A shifter provides a driver selectable input to the transmission indicative of which gear of the transmission is desired by the driver. A shifter may be in the form of a linearly movable shift lever, a rotary knob or a push button, and may be momentary (i.e., biased to return to a “null” position after gear selection) or latching (i.e., physically latched so as to remain at the gear selected). Gear selection typically includes the following gears: gear “P” (park gear), gear “R” (reverse gear), gear “N” (neutral gear), gear “D” (drive gear), and gear “L” (low gear). Optionally, other gear selections may be available.
Shown by way of schematic example in FIGS. 1 through 3 is a latching shifter 10 having a shift lever 12 and a gear cam 24. The shift lever 12 is movable to enable driver selection of a gear from a set of various gears of a mechanical transmission 46, for example gear “P” (park gear) 14, gear “R” (reverse gear) 16, gear “N” (neutral gear) 18, gear “D” (drive gear) 20, and gear “L” (low gear) 22.
A gear cam 24 has formed therein a series of gear notches, including: a park gear notch 24 corresponding to gear “P” 14, a reverse gear notch 26 corresponding to gear “R” 28, a neutral gear notch 30 corresponding to gear “N” 18, a drive gear notch 32 corresponding to gear “D” 20, and a low gear notch 34 corresponding to gear “L” 22. A cam follower 36 is movably connected to a shaft 38 of the shift lever 12 and is biased by a spring 40 disposed within the shaft, wherein the cam follower is biasably received by the gear notch of the selected gear, as shown. A button 42 on the handle 44 of the shift lever 12 is pressed by the driver to shift out of gear “P” (park gear) in electronic association with the driver also placing his/her foot on the brake; wherein the release from/to “P” is effected either mechanically or electronically, as for example described in U.S. Pat. No. 5,489,264, issued on Feb. 6, 1996.
As shown at FIG. 3, the cam follower 36 is seatable into each gear notch, and once seated, thereupon defines the gear selected by the driver. In order to change gears, the cam follower needs to be moved against its biasing out of the gear notch it is in so that the shift lever can move to another selected gear.
In mechanical transmissions, the interface between the shifter and the gears of the transmission is mechanical, so the gear selected by the driver via movement of the shift lever is necessarily the same gear the transmission is in.
However, in shift-by-wire transmissions, the interface between the shifter and the gears of the transmission are electronic, so it is possible for the shifter to be at one gear, while, in fact, the electronics of the transmission, for various reasons related to the vehicle operation and programming, has placed the transmission in another gear than that of the shifter. In such a circumstance, the driver may erroneously infer the transmission is in his/her selected gear, as indicated by the physical position of the shift lever, yet, in fact, the electronics of the transmission has electronically shifted to another gear.
The following Table I provides typical cases of shift override by the electronics of an electronic transmission with respect to shifting from a gear that has already been selected by the driver.
TABLE IFrom DriverTo AutoTo AutoSelectedSelectedSelectedTo AutoTo AutoGearGear PGear RSelected Gear NSelected Gear DPNeverNeverNeverRPossiblePossibleNeverNPossibleNeverNeverDPossibleNeverPossible
The electronic interface of the shifter for a shift-by-wire transmission is accomplished in the prior art utilizing numerous techniques which provide electronic sensing of the position of the shift lever so that the electronics are aware of the gear selection by the diver, as for example utilizing a plurality hall switches or a plurality analog sensors with varying gains (as is common with respect to throttle pedal sensors).
Most prior art shift-by-wire transmission shifters are momentary in order to accommodate numerous overrides that may occur in such transmissions. For example, while driving forward at high speed, a driver's shifter selection of gear “R” may result in the electronics of the transmission selecting, instead, gear “N”. Similarly in the prior art, it is known to provide a convenience feature whereby, in a situation where the vehicle is at rest and driver has shifted to gear “D”, if the driver now turns off the engine, then the electronics of the transmission will automatically select gear “P” before the engine actually turns off. Momentary shifters are compatible with such overrides since the shifter is normally in a “null” or “monostable” position.
However, a problem is presented for shift-by-wire transmissions if a latching shifter is to be used, since a potential conflict may arise in a situation where the shifter's gear selection does not physically match the gear selected by the electronics of the shift-by-wire transmission. In the example above in which a driver turns the engine off while the shift lever is in the drive gear notch, and the electronics of the transmission has electronically selected gear “P” in response to the engine shut off, the problem arises that the latching shifter will remain physically in the drive gear notch. Not only would this cause confusion to the driver, but it presents a problem as to what the driver should do at the commencement of the next engine ignition cycle. The driver may want to shift from gear “P” to gear “D”, but the shift lever is already in drive gear notch! The solution might be to require the driver to first move the shift lever back to the park gear notch and then to the drive gear notch, but that would negate the potential convenience that was originally intended.
Consider next the situation in which a motor vehicle spins out of control and begins travelling backward at high speed. In a mechanical transmission, this situation would result in an engine stall. But, in a shift-by-wire transmission, the electronics of the transmission would attempt to shift the transmission to gear “N” in order to avoid an engine stall. However, this autonomous electronic shifting presents a potential problem with latching shifters, since although the transmission is actually in gear “N”, the shifter remains physically in the drive gear notch. In such a situation, what happens next is a question. If the driver wants to reselect gear “D”, he/she is left with the confusion of the shift lever being already in the drive gear notch! One potential “solution” could be to require the driver to first move the shift lever to the neutral gear notch and then back to the drive gear notch, but this is still confusing.
In the prior art, Jaguar Cars Ltd. of Coventry, England (hereafter simply Jaguar) has addressed some of these latching shift lever concerns. The Jaguar shifter utilizes a motor to move the shifter, in the form of a knob, back to the gear “P” selection, although the motor is not designed to move the shifter to an intermediate position or to move to any other position. In fact, the motor's only capability is to address the operational scenario in which the engine is turned off while the shifter is physically in gear “D” or gear “R”, but the electronics of the transmission has selected gear “P”. Jaguar also has utilized a lockout mechanism, wherein in certain circumstances of operation, for example, the shifter may be permitted to move from gear “D” to gear “N”, but thereupon the shifter is locked in place at gear “N”. Note that the Jaguar latching shifter does not address the operational circumstance in which the electronics of the transmission has selected gear “N”, yet the shifter is, in fact, physically at gear “D”. In this regard, Jaguar simply indicates this untoward condition by a flashing “D” that is viewable by the driver, and meant to convey information to the driver that the transmission is actually in gear “N”, not gear “D”.
Accordingly, it is a significant challenge to motor vehicle engineering to mate a latching shifter with a shift-by-wire transmission, in that in such an arrangement the problem is confusion of the driver with respect to the drive gear notch position of the shift lever when the transmission has autonomously shifted to gear “P” or gear “N” for pre-programmed operational reasons.